After 5 hours of drying, a porous solid under constant drying conditions. Its moisture content was reduced from 30 to 12 lb H2O/lb dry solid. The critical moisture content was found to be 18 lb H2O/lb dry solid and the equilibrium moisture is 5 lb H2O dry solid. The drying rate during the falling rate period is a straight line. What is the Rate at Critical Point?

In the dryer models for a belt dryer with throughcirculation and a direct-heat rotary dryer, is the rate of drying based on heat transfer or mass transfer? Why?

Under what drying conditions is the moisture-evaporation temperature equal to the wet-bulb temperature?

The following data were obtained in a drying test of a solid food placed in a laboratory tray dryer. Find the critical moisture content, the critical drying rate, the drying time in the constant-rate period, the time necessary for the moisture to fall from the critical moisture content to 70% (wet basis), and the amount of water that was removed from the product in the drying test. It is given that the initial moisture content is 85% wet basis, the bulk density of the solid is 50 kg dry solids /m², and the thickness of the solid in the tray is 3 cm. Time Weight (kg) (h) 0 3.000 0.5 2.500 1 2.000 Exercises 265 (continued) Time Weight (kg) (h) 2 1.600 3 1.500 4 1.435 6 1.420 8 1.410 1.405 1.400 1.400 10 12 13

Deterioration must be strictly avoided when drying pharmaceutical products. Furthermore, such products are often produced from a nonaqueous solvent such as ethanol, methanol, acetone, etc. Explain why a closed-cycle spray dryer using nitrogen is frequently a good choice of dryer